scalar: remove stale config values

These config values were added in the original Scalar contribution, d0feac4e8c (scalar: 'register' sets recommended config and starts maintenance, 2021-12-03), but were never fully checked for validity in the upstream Git project. At the time, Scalar was only intended for the contrib/ directory so did not have as rigorous of an investigation. Each config option has its own justification for removal: * core.preloadIndex: This value is true by default, now. Removing this causes some changes required to the tests that checked this config value. Use gui.gcwarning=false instead. * core.fscache: This config does not exist in the core Git project, but is instead a config option for a Git for Windows feature. * core.multiPackIndex: This config value is now enabled by default, so does not need to be called out specifically. It was originally included to make sure the background maintenance that created multi-pack-indexes would result in the expected performance improvements. * credential.validate: This option is not something specific to Git but instead an older version of Git Credential Manager for Windows. That software was replaced several years ago by the cross-platform Git Credential Manger so this option is no longer needed to help users who were on that older software. * pack.useSparse=true: This value is now Git's default as of de3a864114 (config: set pack.useSparse=true by default, 2020-03-20) so we don't need it set by Scalar. Signed-off-by: Derrick Stolee <stolee@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>